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How we know the Cosmos is 13.8 billion years old?

The Cosmos is 13.8 billion years old, going back to the hot Big Bang. But was that really the beginning, and is that actually its age?


·         If we calculate from the start of the hot Big Bang, we learn that the Cosmos is 13.8 billion years old, with only a very miniscule (~1%) degree of uncertainty.

·         But what endows us the right to call the start of the hot Big Bang “the beginning,” particularly if we now can say with certain amount of certainty that an epoch of cosmic inflation preceded it?

·         The truth is that we have to make choices, and the start of the hot Big Bang is one of the earliest things we can be sure about. Here’s what the “age of the Universe” in fact means.

As per the theory of the hot Big Bang, the Universe had a commencement. Initially known as “a day without a yesterday,” this is one of the most contentious, philosophically staggering pieces of information we’ve come to accept as part and parcel of the scientific history of our Cosmos. Many detractors will dismiss it as being too much in-line with certain religious texts, while others — maybe more justifiably — note that in the contemporary context of cosmic inflation, the hot Big Bang only happened as the aftermath of a previous epoch.

And still, if you ask any astrophysicist or cosmologist who’s well-versed in the scientific story of our beginnings “How old is our Cosmos?” you always get the same reply: 13.8 billion years. Why is this, and when actually do we start counting? That’s what Denis Gaudet desired to know, having written in to ask:

“Why do you start counting the age of the universe after 380,000 years have elapsed after the Big Bang?”The time “380,000 years after the Big Bang” is of particular importance, but only few people mark that as the beginning of the Cosmos; it is the beginning of something significant, however. Here’s what we can say about how old our Universe actually is.The first thing you need to appreciate is that there are two different ways of measuring the age of the Cosmos since the start of the hot Big Bang.

1.       We can find “the oldest thing we know how to measure its age” and infer that the Universe must be at least that old.

2.       We can use what we are aware about the theory that governs the Cosmos, general relativity, as well as our understanding of what the Universe is made out of plus how fast it’s expanding today to calculate how long it’s been post the start of the hot Big Bang.

The first method isn’t precisely a measurement of how old the Universe is, but rather just a sanity check: the Cosmos cannot be younger than the things in it, so when we discover things in it and measure their ages, we infer that the Universe must be at least that old.As astrophysics and cosmology grew out of the far older sciences of physicsand astronomy, it should come as no surprise that one of the things we’ve become very good at knowing the ages of are stars and large populations of stars.


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